In an update on J. K. Rowling’s website, the author makes quite clear that plans to publish a physical copy of a Harry Potter Encyclopaedia are not on the horizon. The entry, updated May 2012, reads:

What about the Harry Potter Encyclopaedia?

Updated May 2012

I have been enjoying sharing information about Harry’s world on Pottermore for free, and don’t have any firm plans to publish it in book form.

This statement replaces the April update on her website where she said “For a long time I have been promising an encyclopaedia of Harry’s world, and I have started work on this now ’ some of it forms the new content in Pottermore. It is likely to be a time-consuming job, but when finished I shall donate all royalties to charity.”